crypto: dh - Remove the unused function dh_safe_prime_dh_alg()

Fix the following W=1 kernel warnings:

crypto/dh.c:311:31: warning: unused function 'dh_safe_prime_dh_alg'
[-Wunused-function]

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>